In recent years, out of concern for the global environment, lightweight and high-strength materials has been demanded for use as structural metal materials.
For example, in automobiles, there has been an increased demand for safety of automobile bodies, there is an increased demand for decreases in weight and increases in the strength of automobile parts, and the development of automobile parts is being advanced from the standpoints of increasing fuel efficiency and increasing safety in collisions. In order to cope with such demands, high-strength materials having a significantly higher strength level than in the past such as high tensile strength steel sheet having a tensile strength of at least 780 MPa or even at least 900 MPa are much used.
Along with an increase in the strength of such materials, there has been a rethinking of the structure of automobile parts. For example, in order to have applications to a variety of automobile parts, there is a strong demand for development of bending techniques which can perform high precision working of products having a widely varying bent shape, such as products having continuous bending in which the bending direction varies two-dimensionally as in S-bending, or continuous bending in which the bending direction varies three-dimensionally.
In order to meet such a demand, in Patent Document 1, for example, an invention which relates to a method of bending while performing heat treatment of a metal pipe or the like is disclosed in which the end portion of a member being worked such as a metal pipe is held by a rotatable arm, a portion of the metal material is heated by a heating device while the heated portion is suitably moved to produce bending deformation, and then cooling is carried out. Patent Document 2 discloses an invention related to a method of bending while performing heat treatment of a metal pipe or the like by applying a twisting force and a bending force to a heated portion of a metal pipe and carrying out bending deformation while twisting the metal pipe.
Taking into consideration decreases in the weight of products formed by bending, it is desirable to set their tensile strength to at least 900 MPa and more preferably to at least 1300 MPa. In this case, as disclosed in Patent Documents 1 and 2, bending is carried out on a starting material in the form of a pipe having a tensile strength of around 500-700 MPa, after which the strength is increased by heat treatment to manufacture a bent product having a desired high strength.
However, the inventions disclosed in Patent Documents 1 and 2 both relate to working methods which fall into the category of so-called grip bending. In order to carry out either invention, it is necessary to hold the end portion of a member being worked with a rotatable arm. Therefore, the member being worked cannot be fed at a high speed, and each time gripping of the member being worked by the arm is repeated, it is necessary to return the arm, so the feed speed of the member being worked greatly fluctuates, it becomes difficult to control the cooling rate in a complicated manner, and a desired hardening accuracy cannot be achieved.
It becomes necessary to control the heating and cooling rate in a complicated manner, but even if doing so, non-uniform strains develop, and a desired hardening accuracy cannot be guaranteed. Therefore, variations in the bent shape develop, and when dealing with a high-strength material, delayed fracture due to residual stresses occurs, and it is difficult to manufacture an automobile part requiring high reliability.
In Patent Document 3, an invention is disclosed which relates to a bending apparatus with high-frequency heating. A material being worked which is supported by a support means while being fed from an upstream side towards a downstream side by a feed device is subjected to bending on the downstream side of the support apparatus by compression bending rollers which are supported so as to be able to move three-dimensionally. According to the bending apparatus with high-frequency heating disclosed in Patent Document 3, the compression bending rollers straddle the material to be worked and move to opposite sides of the material being worked and perform bending while contacting the side surfaces. Therefore, even when performing continuous bending such as S-bending in which the bending direction varies two-dimensionally, there is no need to carry out a setup operation in which the material being worked is rotated by 180°, and bending can be efficiently carried out.
However, in the bending apparatus with high-frequency heating disclosed in Patent Document 3, there is no means for clamping both side surfaces of the material being worked. Therefore, deformation caused by residual stress due to cooling after high frequency heating easily occur, which makes it difficult to guarantee a prescribed dimensional accuracy. In addition, there are limitations on the working speed and it is difficult to increase the degree of working.
Patent Document 4 discloses an invention related to a bending apparatus. In place of the compression bending rollers of the above-described grip bending or bending apparatus with high-frequency heating, it includes a fixed die which is stationary and a movable gyro-die which can move three-dimensionally and is spaced from the fixed guide. In addition, a heating means is provided so as to heat a metal material being worked to a temperature corresponding to the curvature during bending by the movable gyro-die.
The fixed die and the movable gyro-die which constitute the bending apparatus disclosed in Patent Document 4 do not hold a metal material which is a material being worked so that it can rotate. Therefore, the surfaces of both the fixed die and the movable gyro-die easily develop seizing scratches when holding the metal material. The bending apparatus disclosed in Patent Document 4 supplies a cooling fluid to the fixed die and the movable gyro-die in order to prevent a decrease in the strength of the die and to prevent a decrease in working accuracy due to thermal expansion. However, supply of the cooling fluid is not for the purpose of quenching heat treatment of the metal material being bent, so it is not possible to manufacture a bent product having a high strength such as at least 900 MPa by quenching.
The bending apparatus disclosed in Patent Document 4 is not intended to obtain a high-strength metal material by using a low-strength metal pipe as a starting material and then increasing its strength by quenching after hot working. In addition, due to heating of the metal material, the surface of the movable gyro-die easily develops seizing scratches, so further improvements as a bending apparatus is needed.
In Patent Document 5, the present applicants disclosed an invention which manufactures a bent product with high efficiency while maintaining a sufficient bending accuracy using a bending method which carries out bending downstream of a support means while a metal material held by the support means is fed by a feed device from the upstream side towards the downstream side. When manufacturing a bent product with this method, a portion of the metal material is heated to a temperature range in which it can be locally quenched by a heating means for the metal material downstream of the support means, and the position of a movable roller die which is disposed downstream of the heating means and has at least one set of roll pairs which can support the metal material while feeding it is varied two-dimensionally or three-dimensionally to apply a bending moment to a portion of the metal material being fed which was heated by the heating means and carry out bending.
Patent Document 1: JP 50-59263 A
Patent Document 2: Japanese Patent 2816000
Patent Document 3: JP 2000-158048 A
Patent Document 4: Japanese Patent 3195083
Patent Document 5: WO2006/093006